Here’s an interesting feature that was planned for Firefox 3.6, but not all the kinks have been worked out so it is disabled by default. The idea behind this feature is if you have already scrolled through a page a couple times before you are more than likely looking for something in particular or trying to reach a particular part of the page. By default, the scroll speed via the mouse wheel is the same no matter how many times before you have successively scroll through the page prior. This tweak will allow you to set a minimum number of times to scroll through the page before the scrolling acceleration is ‘triggered’ and then how fast the scrolling is accelerated.

Caution:I mentioned earlier there are some kinks that Mozilla has yet to work out. This involves possible conflicts with mouse driver based acceleration.

To enable to Scroll Acceleration/Setting the Trigger Point:

• In a new tab type about:config in the address bar
• Filter for mousewheel.acceleration.start
• Double-click the entry and enter the number of successive mouse scrolls to trigger the Scroll Acceleration. Example: a setting of 3 will trigger the Scroll Acceleration after the 3rd successive mouse scroll on that page.

To set the Scroll Acceleration Speed:

• In a new tab type about:config in the address bar
• Filter for mousewheel.acceleration.factor
• Double-click the entry and enter the number equal to the number times faster the scroll speed is going to be. Note: The default is 10, but Percy had found 5 to work better.

Source: Mozilla Links